AUSTIN, Texas (Reuters) - Fear and failure haunted Lewis Hamilton at the U.S. Grand Prix on Sunday, even as he sped towards victory and membership of Formula One's super-elite club of 50 race winners.

When the triple world champion was informed that he had joined greats Michael Schumacher and Alain Prost as only the third driver to reach the half century, the Briton sounded surprised.

Most of his attention, on what to all appearances was a dominant afternoon in the Texas sunshine, was spent fretting about a possible engine failure dealing another devastating blow to his battered title hopes.

"In the cockpit all I could think about was the car making it to the end," Hamilton told reporters after the champagne had been sprayed, team photographs taken and autographs signed. "Nothing else.

"It was the whole race. It was the longest afternoon that I've had probably that I can remember really in my career, just hoping that the car makes it to the end. Gearbox. You're feeling all the vibrations of the engine through the rev range and gearshifts.

"Every single lap I was thinking something might happen ... when I get on that backstraight I'm petrified. It's got a beautiful ring to it, but it had a beautiful ring in Malaysia. I was honestly just haunted by that.

"When I came across the line I was so happy, just rubbing the cockpit, thank you for getting me there."

In Malaysia two races earlier, Hamilton had been enjoying a similarly commanding afternoon when the engine suddenly failed and 25 points disappeared in smoke.

With them, he would now be leading the championship rather than fighting 26 points behind German team mate and title favourite Nico Rosberg with three races remaining.

The nightmare of Sepang cast a dark cloud over the subsequent race in Japan and still haunted Hamilton even as he took pole position in Texas.

"I was trying to do fewer gear shifts through a lap to try and save, make the gearbox go longer than perhaps it wants to," he said.

"With the engine I wasn't doing 100 percent throttle down the straight, I was pulling it back to 90 in some scenarios just to try not to stretch it, because for whatever reason it's more likely to happen on my car."

Hamilton has had two retirements this year to Rosberg's one, and that was due to a collision between the pair, but the Briton has had more setbacks -- particularly early on when Rosberg won the opening four rounds.

"I'm hoping that for these last four I have 100 percent reliability," he said. "That will be a real breath of fresh air."